If this is the book by Paul Lund and Harry Ludlam, it's not that rare at all and pretty easy to find. Amazon.com is currently showing at least 17 copies starting at three dollars or so up to $106.27. Paid $8.00 CAD for my copy back in the mid-nineties but it is a first edition even if well worn.
http://www.amazon.com/Night-U-Boats-...4351729&sr=8-1
Books can be frightingly expensive, I ended up paying over $600.00 CAD for the Arthur Marder five-volume series
From Dreadnaught to Scapa Flow and that after shopping around for years.